A discussion of the evolution and observable consequences of a network of cosmic strings is given. A simple model for the evolution of the string network is presented and related to the statistical mechanics of string networks. The model predicts the long-string density throughout the history of the Universe from a single parameter, which we calculate in radiation-era simulations. The statistical mechanics arguments indicate a particular "thermal" form for the spectrum of loops chopped off the network. Detailed numerical simulations of string networks in expanding backgrounds are performed to test the model. Consequences for large-scale structure, the microwave and gravity-wave backgrounds, nucleosynthesis, and gravitational lensing are calculated.

• Received 17 March 1989

DOI:https://doi.org/10.1103/PhysRevD.40.973